Coking, ageing and regeneration of zeolites: VI. Comparison of the Rates of Coke Oxidation of HY, H-Mordenite and HZSM-5

Abstract

The oxidation rates of the carbonaceous deposits (“coke”) formed on three zeolites, HY, H-mordenite and HZSM-5, during n-heptane cracking were compared for various coke contents. Whatever the zeolite, oxidation occurs through successive steps with intermediate formation of oxygenated compounds. The oxidation rate depends very little on the coke content but is very dependent on the zeolite; in particular, it is higher on HY and H-mordenite than on HZSM-5. The differences in the composition and in the location of coke (in the porosity or on the outer surface) cannot explain this. The low oxidation rate found on HZSM-5 is due to the fact that there is limited contact between oxygen and the carbonaceous compounds because the oxygen cannot diffuse through the channel intersections occupied by these compounds. With HY, oxygen can circulate freely in the supercages occupied by coke. With H-mordenite, for which coke is deposited at the mouth of the large channels, oxygen can enter through the small channels into almost the entire pore volume. Coke oxidation, like coking and ageing, is therefore a shape-selective process.